In this photo provided by the United Launch Alliance, a ULA Atlas V rocket lifts off from Cape Canaveral Air Force Station in Cape Canaveral, Fla. on Wednesday, May 20, 2015. The rocket carried the X-37B... (United Launch Alliance via AP)

(Newser)
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The Air Force owns two X-37B space planes, and one of them is finally back on the ground after a nearly 2-year-long secret mission. While the what is unknown, the how long is clear: 718 days in orbit, reports Space.com, with the unmanned plane landing Sunday morning at NASA's Kennedy Space Center in Florida—and flying into the record books. It was the Air Force's fourth such mission since April 2010 and the longest to date. A fifth is planned for 2017, reports Reuters. There's plenty of speculation about what the 29-foot-long plane, which looks like a mini-space shuttle, is up to up there.

Reuters cites one group's belief that the secrecy indicates intelligence-related hardware tests are being conducted. An Air Force fact sheet offers up this much: "The primary objectives of the X-37B are twofold; reusable spacecraft technologies for America’s future in space and operating experiments which can be returned to, and examined, on Earth." One cool fact: Though its mission is classified, its location isn't, thanks to what Space.com refers to as "keen-eyed amateur astronomers." The site offers a satellite tracker when missions are active, and notes the X-37B "usually looks like a star of middling brightness moving across the sky."